Heretofore, in this type of optical disk apparatus, the optical head is moved as far as the desired recording track to bring about a just-tracking state, and the desired data or information is subsequently recorded or reproduced. At this time, movement of the optical head is terminated in a shorter time to make data recording or reproduction in a shorter time as a whole.
Thus, in the conventional optical disk apparatus, the optical head is driven so that, as shown in FIG. 1, it is accelerated promptly so that its speed of movement becomes zero on the recording track of destination, referred to hereinafter as the target track.
That is, the distance X traversed by a light spot, as shown by the following formula (1) EQU X=N.multidot.Tp (1)
is obtained, wherein N stands for the number of recording tracks from the recording track being scanned by the light spot emanating from the optical head as far as the target track and Tp the pitch of the recording tracks, and the target speed of movement proportionate to .times..sup.1/2 is set, for driving the optical head.
In addition, the number of tracks traversed by the light spot during movement of the optical head is counted (this counting is referred to hereinafter as traverse counting) and the traversed distance X is sequentially corrected to compensate for the target speed of movement of the optical head so that the speed of movement of the optical head becomes zero on the target track.
When the light spot has moved as far as the target track, the optical head is tracking-controlled on the basis of the track error signal S.sub.TE, as shown in FIG. 2, which is obtained by the medium of the optical head, in such a manner that the desired information may be positively recorded or reproduced with the light spot in the state of just-tracking of the target track.
Meanwhile, in this type of optical disk apparatus, if traverse counting can be made with a high degree of precision, the speed of movement of the optical head can be correctly controlled so as to be equal to zero on the target track, and hence the information can be recorded or reproduced within a correspondingly shorter time.
For this reason, there is proposed in the Japanese Patent application No. 207542/1987 or the like an arrangement in which, as shown for example in FIG. 3, sequentially circulating simple identification signals are recorded on consecutive recording tracks on the optical disk 1, so that, even when the light spot is moved across a given recording track, the position of the recording track may be discriminated.
The optical disk 1 shown in FIG. 3 is divided circumferentially into plural sectors at a predetermined angular interval, and a control recording area AR.sub.C is formed at the terminal part of each sector.
In a forward area AR.sub.S of the control recording region AR.sub.C, there are formed a pit Q.sub.B on a track center TRC of each recording track, and pits Q.sub.A and Q.sub.C, each with a predetermined offset from each track center TRC. Tracking error signals are obtained on the basis of these pits Q.sub.A, Q.sub.B and Q.sub.C.
On the other hand, in a forward zone of the control recording region AR.sub.C, there is formed a traverse area AR.sub.T, in which recording patterns sequentially circulated at a period of 16 tracks are formed using first pits Q.sub.E1 and second pits Q.sub.E2.
More specifically, the traverse area AR.sub.T is divided into eight parts at a predetermined interval D, and the first pits Q.sub.E1 are shifted at intervals of the four consecutive record tracks towards the rear by a distance equal to the interval D. On the other hand, the second pits Q.sub.E2 are shifted each by a distance equal to the interval D towards the rear or towards the front at periods of the four consecutive record tracks. This permits the position of the recording track being traversed by the light spot to be discriminated, even during the time that the light spot is moving across the recording track, on the basis of the generation timing of the reproducing signals of the first pits Q.sub.E1 and the second pits Q.sub.E2.
Hence, the target speed of movement may be obtained with high precision when the target speed of movement is corrected on the basis of the results of discrimination.
However, in practice, the recording tracks are formed on the optical disk apparatus at a higher density of the track pitch of, for example, the order of 1.6 (.mu.m). For this reason, in the arrangement of the optical head as a whole, a fine adjustment actuator, loaded with an objective lens, is provided on a gross adjustment actuator adapted to travel radially of the optical disk, in such a manner that the movement of the optical head can be completed in a shorter time and tracking control can be made a with high degree of precision.
For this reason, when the gross adjustment actuator is driven to cause a high-speed movement of the optical head, oscillations of the objective lens loaded on the fine adjustment actuator occur unavoidably, with the result that, when the speed of movement of the optical head on the target track becomes zero, the speed of movement of the light spot formed on the optical disk 1 is not exactly equal to zero.
In this case, when the optical head is switched to tracking control at such a timing that it is moving along the direction shown by an arrow mark a in FIG. 2, with a predetermined offset from the track center P.sub.M of the target track, as shown at P in FIG. 2, the light spot movement is accelerated.
In this case, when the light spot is moved across the track center P.sub.M to a region on the negative side of the tracking error signal S.sub.TE, the light spot movement is decelerated.
However, when the light spot is accelerated in this manner, sufficient deceleration cannot be achieved within the range up to the position of the negative maximum P.sub.MAX of the tracking error signal S.sub.TE, such that the light spot is moved past the position P.sub.MAX as far as the next recording track.
Moreover, since the tracking error signal S.sub.TE repeats itself at a track pitch in the form of a sinusoidal wave, when the light spot is moved past the maximum value position P.sub.MAX, it may occur that the light spot is moved up to the maximum point of possible movement stroke of the fine adjustment actuator, with the result that the light spot makes a run away as far as a recording track several hundreds of tracks off the track center P.sub.M of the target track.
In such case, it becomes necessary to count the number of the recording tracks from the recording track, to which the light spot has reached by run away, as far as the target track, and to return the light spot back to the desired target track, with the consequence that a correspondingly long time is expended in shifting the optical head and hence it becomes difficult to record or reproduce the information within a shorter time on the whole.
For overcoming such inconvenience, there is known a method consisting in controlling the timing of switching to tracking control in dependence upon the speed of movement of the light spot.
However, in practice, in the optical disk apparatus, offset or defects of the optical disk or the effect of disturbances extraneous to the optical disk apparatus occur unavoidably, such that the occurrence of run away of the light spot may not be prevented completely by controlling the timing of switching to tracking control of the light spot.
In view of the foregoing, it is an object of the present invention to provide an optical disk apparatus wherein the occurrence of run away of the light spot is prevented and the light spot may be moved in a short time as far as the desired recording track.